posted 04-04-2017 02:17 PM               

NASA's Cassini Mission Prepares for 'Grand Finale' at Saturn

NASA's Cassini spacecraft, in orbit around Saturn since 2004, is about to begin the final chapter of its remarkable story. On Wednesday, April 26, the spacecraft will make the first in a series of dives through the 1,500-mile-wide (2,400-kilometer) gap between Saturn and its rings as part of the mission's grand finale.

"No spacecraft has ever gone through the unique region that we'll attempt to boldly cross 22 times," said Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters in Washington. "What we learn from Cassini's daring final orbits will further our understanding of how giant planets, and planetary systems everywhere, form and evolve. This is truly discovery in action to the very end."

During its time at Saturn, Cassini has made numerous dramatic discoveries, including a global ocean that showed indications of hydrothermal activity within the icy moon Enceladus, and liquid methane seas on its moon Titan.

Now 20 years since launching from Earth, and after 13 years orbiting the ringed planet, Cassini is running low on fuel. In 2010, NASA decided to end the mission with a purposeful plunge into Saturn this year in order to protect and preserve the planet's moons for future exploration – especially the potentially habitable Enceladus.

But the beginning of the end for Cassini is, in many ways, like a whole new mission. Using expertise gained over the mission's many years, Cassini engineers designed a flight plan that will maximize the scientific value of sending the spacecraft toward its fateful plunge into the planet on Sept. 15. As it ticks off its terminal orbits during the next five months, the mission will rack up an impressive list of scientific achievements.

"This planned conclusion for Cassini's journey was far and away the preferred choice for the mission's scientists," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "Cassini will make some of its most extraordinary observations at the end of its long life."

The mission team hopes to gain powerful insights into the planet's internal structure and the origins of the rings, obtain the first-ever sampling of Saturn's atmosphere and particles coming from the main rings, and capture the closest-ever views of Saturn's clouds and inner rings. The team currently is making final checks on the list of commands the robotic probe will follow to carry out its science observations, called a sequence, as it begins the finale. That sequence is scheduled to be uploaded to the spacecraft on Tuesday, April 11.

Cassini will transition to its grand finale orbits, with a last close flyby of Saturn's giant moon Titan, on Saturday, April 22. As it has many times over the course of the mission, Titan's gravity will bend Cassini's flight path. Cassini's orbit then will shrink so that instead of making its closest approach to Saturn just outside the rings, it will begin passing between the planet and the inner edge of its rings.

"Based on our best models, we expect the gap to be clear of particles large enough to damage the spacecraft. But we're also being cautious by using our large antenna as a shield on the first pass, as we determine whether it's safe to expose the science instruments to that environment on future passes," said Earl Maize, Cassini project manager at JPL. "Certainly there are some unknowns, but that's one of the reasons we're doing this kind of daring exploration at the end of the mission."

In mid-September, following a distant encounter with Titan, the spacecraft's path will be bent so that it dives into the planet. When Cassini makes its final plunge into Saturn's atmosphere on Sept. 15, it will send data from several instruments – most notably, data on the atmosphere's composition – until its signal is lost.

"Cassini's grand finale is so much more than a final plunge," said Spilker. "It's a thrilling final chapter for our intrepid spacecraft, and so scientifically rich that it was the clear and obvious choice for how to end the mission."

posted 04-26-2017 04:37 AM               

Cassini's First Grand Finale Dive: Milestones

NASA's Cassini spacecraft is set to make its first dive through the narrow gap between Saturn and its rings on April 26, 2017. Because that gap is a region no spacecraft has ever explored, Cassini will use its dish-shaped high-gain antenna (13 feet or 4 meters across) as a protective shield while passing through the ring plane. No particles larger than smoke particles are expected, but the precautionary measure is being taken on the first dive.

The Cassini team will use data collected by one of the spacecraft's science instruments (the Radio and Plasma Wave Subsystem, or RPWS) to ascertain the size and density of ring particles in the gap in advance of future dives. As a result of its antenna-forward orientation, the spacecraft will be out of contact with Earth during the dive.

Below is a list of milestones expected to occur during the event, if all goes as planned:

• 5 p.m. PDT (8 p.m. EDT) on April 25: Cassini is approaching Saturn over the planet's northern hemisphere in advance of its first of 22 planned dives through the gap between the planet and its rings.

• 1:34 a.m. PDT (4:34 a.m. EDT) on April 26: As it passes from north to south over Saturn, Cassini begins a 14-minute turn to point its high-gain antenna into the direction of oncoming ring particles. In this orientation, the antenna acts as a protective shield for Cassini's instruments and engineering systems.

• 2 a.m. PDT (5 a.m. EDT) on April 26: Cassini crosses the ring plane during its dive between the rings and Saturn. The spacecraft's science instruments are collecting data, but Cassini is not in contact with Earth at this time.

• No earlier than around midnight PDT on April 26 (3 a.m. EDT on April 27): Earth has its first opportunity to regain contact with Cassini as the giant, 230-foot (70-meter) Deep Space Network antenna at Goldstone, California, listens for the spacecraft's radio signal.

• Likely no earlier than 12:30 a.m. PDT (3:30 a.m. EDT) on April 27: Images are scheduled to become available from the spacecraft.

As Cassini engineers monitor the status of the spacecraft, updates to these milestones will be added here.

posted 04-27-2017 02:52 AM               

NASA Spacecraft Dives Between Saturn and Its Rings

NASA's Cassini spacecraft is back in contact with Earth after its successful first-ever dive through the narrow gap between the planet Saturn and its rings on April 26, 2017.

The spacecraft is in the process of beaming back science and engineering data collected during its passage, via NASA's Deep Space Network Goldstone Complex in California's Mojave Desert. The DSN acquired Cassini's signal at 11:56 p.m. PDT on April 26, 2017 (2:56 a.m. EDT on April 27) and data began flowing at 12:01 a.m. PDT (3:01 a.m. EDT) on April 27.

Above: These unprocessed images show features in Saturn's atmosphere from closer than ever before. The view was captured by NASA's Cassini spacecraft during its first Grand Finale dive past the planet, April 26, 2017. (NASA/JPL-Caltech/Space Science Institute)

"In the grandest tradition of exploration, NASA's Cassini spacecraft has once again blazed a trail, showing us new wonders and demonstrating where our curiosity can take us if we dare," said Jim Green, director of the Planetary Science Division at NASA Headquarters in Washington.

As it dove through the gap, Cassini came within about 1,900 miles (3,000 kilometers) of Saturn's cloud tops (where the air pressure is 1 bar — comparable to the atmospheric pressure of Earth at sea level) and within about 200 miles (300 kilometers) of the innermost visible edge of the rings.

While mission managers were confident Cassini would pass through the gap successfully, they took extra precautions with this first dive, as the region had never been explored.

"No spacecraft has ever been this close to Saturn before. We could only rely on predictions, based on our experience with Saturn's other rings, of what we thought this gap between the rings and Saturn would be like," said Cassini Project Manager Earl Maize of NASA's Jet Propulsion Laboratory in Pasadena, California. "I am delighted to report that Cassini shot through the gap just as we planned and has come out the other side in excellent shape."

The gap between the rings and the top of Saturn's atmosphere is about 1,500 miles (2,000 kilometers) wide. The best models for the region suggested that if there were ring particles in the area where Cassini crossed the ring plane, they would be tiny, on the scale of smoke particles. The spacecraft zipped through this region at speeds of about 77,000 mph (124,000 kph) relative to the planet, so small particles hitting a sensitive area could potentially have disabled the spacecraft.

As a protective measure, the spacecraft used its large, dish-shaped high-gain antenna (13 feet or 4 meters across) as a shield, orienting it in the direction of oncoming ring particles. This meant that the spacecraft was out of contact with Earth during the ring-plane crossing, which took place at 2 a.m. PDT (5 a.m. EDT) on April 26. Cassini was programmed to collect science data while close to the planet and turn toward Earth to make contact about 20 hours after the crossing.

Cassini's next dive through the gap is scheduled for May 2.

posted 05-01-2017 08:43 PM               

Cassini Finds 'The Big Empty' Close to Saturn

As NASA's Cassini spacecraft prepares to shoot the narrow gap between Saturn and its rings for the second time in its Grand Finale, Cassini engineers are delighted, while ring scientists are puzzled, that the region appears to be relatively dust-free. This assessment is based on data Cassini collected during its first dive through the region on April 26.

With this information in hand, the Cassini team will now move forward with its preferred plan of science observations.

"The region between the rings and Saturn is 'the big empty,' apparently," said Cassini Project Manager Earl Maize of NASA's Jet Propulsion Laboratory in Pasadena, California. "Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected."

A dustier environment in the gap might have meant the spacecraft's saucer-shaped main antenna would be needed as a shield during most future dives through the ring plane. This would have forced changes to how and when Cassini's instruments would be able to make observations. Fortunately, it appears that the "plan B" option is no longer needed. (There are 21 dives remaining. Four of them pass through the innermost fringes of Saturn's rings, necessitating that the antenna be used as a shield on those orbits.)

Based on images from Cassini, models of the ring particle environment in the approximately 1,200-mile-wide (2,000-kilometer-wide) region between Saturn and its rings suggested the area would not have large particles that would pose a danger to the spacecraft.

But because no spacecraft had ever passed through the region before, Cassini engineers oriented the spacecraft so that its 13-foot-wide (4-meter-wide) antenna pointed in the direction of oncoming ring particles, shielding its delicate instruments as a protective measure during its April 26 dive.

Cassini's Radio and Plasma Wave Science (RPWS) instrument was one of two science instruments with sensors that poke out from the protective shield of the antenna (the other being Cassini's magnetometer). RPWS detected the hits of hundreds of ring particles per second when it crossed the ring plane just outside of Saturn's main rings, but only detected a few pings on April 26.

When RPWS data are converted to an audio format, dust particles hitting the instrument's antennas sound like pops and cracks, covering up the usual whistles and squeaks of waves in the charged particle environment that the instrument is designed to detect. The RPWS team expected to hear a lot of pops and cracks on crossing the ring plane inside the gap, but instead, the whistles and squeaks came through surprisingly clearly on April 26.

"It was a bit disorienting — we weren't hearing what we expected to hear," said William Kurth, RPWS team lead at the University of Iowa, Iowa City. "I've listened to our data from the first dive several times and I can probably count on my hands the number of dust particle impacts I hear."

The team's analysis suggests Cassini only encountered a few particles as it crossed the gap — none larger than those in smoke (about 1 micron across).

Cassini will next cross through the ring plane Tuesday, May 2, at 12:38 p.m. PDT (3:38 p.m. EDT) in a region very close to where it passed on the previous dive. During this orbit, in advance of the crossing, Cassini's cameras have been looking closely at the rings; in addition, the spacecraft has rotated (or "rolled") faster than engineers have ever allowed it to before, in order to calibrate the magnetometer. As with the first finale dive, Cassini will be out of contact during closest approach to Saturn, and is scheduled to transmit data from this dive on May 3.

posted 05-03-2017 06:09 PM               

New Movie Shows Cassini's First Dive over Saturn

A new movie sequence of images from NASA's Cassini spacecraft shows the view as the spacecraft swooped over Saturn during the first of its Grand Finale dives between the planet and its rings on April 26.

The movie comprises one hour of observations as the spacecraft moved southward over Saturn. It begins with a view of the swirling vortex at the planet's north pole, then heads past the outer boundary of the hexagon-shaped jet stream and beyond.

"I was surprised to see so many sharp edges along the hexagon's outer boundary and the eye-wall of the polar vortex," said Kunio Sayanagi, an associate of the Cassini imaging team based at Hampton University in Virginia, who helped produce the new movie. "Something must be keeping different latitudes from mixing to maintain those edges," he said.

Toward the end of the movie, the camera frame rotates as the spacecraft reorients to point its large, saucer-shaped antenna in the direction of the spacecraft's motion. The antenna was used as a protective shield during the crossing of Saturn's ring plane.

As the movie frames were captured, the Cassini spacecraft's altitude above the clouds dropped from 45,000 to 4,200 miles (72,400 to 6,700 kilometers). As this occurred, the smallest resolvable features in the atmosphere changed from 5.4 miles (8.7 kilometers) per pixel to 0.5 mile (810 meters) per pixel.

"The images from the first pass were great, but we were conservative with the camera settings. We plan to make updates to our observations for a similar opportunity on June 28 that we think will result in even better views," said Andrew Ingersoll, a member of the Cassini imaging team based at Caltech in Pasadena, California.

posted 08-09-2017 12:34 PM               

Cassini to Begin Final Five Orbits Around Saturn

NASA's Cassini spacecraft will enter new territory in its final mission phase, the Grand Finale, as it prepares to embark on a set of ultra-close passes through Saturn's upper atmosphere with its final five orbits around the planet.

Cassini will make the first of these five passes over Saturn at 12:22 a.m. EDT Monday, Aug. 14. The spacecraft's point of closest approach to Saturn during these passes will be between about 1,010 and 1,060 miles (1,630 and 1,710 kilometers) above Saturn's cloud tops.

The spacecraft is expected to encounter atmosphere dense enough to require the use of its small rocket thrusters to maintain stability – conditions similar to those encountered during many of Cassini's close flybys of Saturn's moon Titan, which has its own dense atmosphere.

"Cassini's Titan flybys prepared us for these rapid passes through Saturn's upper atmosphere," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory (JPL) in California. "Thanks to our past experience, the team is confident that we understand how the spacecraft will behave at the atmospheric densities our models predict."

Maize said the team will consider the Aug. 14 pass nominal if the thrusters operate between 10 and 60 percent of their capability. If the thrusters are forced to work harder – meaning the atmosphere is denser than models predict – engineers will increase the altitude of subsequent orbits. Referred to as a "pop-up maneuver," thrusters will be used to raise the altitude of closest approach on the next passes, likely by about 120 miles (200 kilometers).

If the pop-up maneuver is not needed, and the atmosphere is less dense than expected during the first three passes, engineers may alternately use the "pop-down" option to lower the closest approach altitude of the last two orbits, also likely by about 120 miles (200 kilometers). Doing so would enable Cassini's science instruments, especially the ion and neutral mass spectrometer (INMS), to obtain data on the atmosphere even closer to the planet's cloud tops.

"As it makes these five dips into Saturn, followed by its final plunge, Cassini will become the first Saturn atmospheric probe," said Linda Spilker, Cassini project scientist at JPL. "It's long been a goal in planetary exploration to send a dedicated probe into the atmosphere of Saturn, and we're laying the groundwork for future exploration with this first foray."

Other Cassini instruments will make detailed, high-resolution observations of Saturn's auroras, temperature, and the vortexes at the planet's poles. Its radar will peer deep into the atmosphere to reveal small-scale features as fine as 16 miles (25 kilometers) wide – nearly 100 times smaller than the spacecraft could observe prior to the Grand Finale.

On Sept. 11, a distant encounter with Titan will serve as a gravitational version of a large pop-down maneuver, slowing Cassini's orbit around Saturn and bending its path slightly to send the spacecraft toward its Sept. 15 plunge into the planet.

During the half-orbit plunge, the plan is to have seven Cassini science instruments, including INMS, turned on and reporting measurements in near real time. The spacecraft is expected to reach an altitude where atmospheric density is about twice what it encountered during its final five passes. Once Cassini reaches that point, its thrusters will no longer be able to work against the push of Saturn's atmosphere to keep the spacecraft's antenna pointed toward Earth, and contact will permanently be lost. The spacecraft will break up like a meteor moments later, ending its long and rewarding journey.

posted 08-29-2017 01:16 PM               

Saturn Plunge Nears for Cassini Spacecraft

NASA's Cassini spacecraft is 18 days from its mission-ending dive into the atmosphere of Saturn. Its fateful plunge on Sept. 15 is a foregone conclusion — an April 22 gravitational kick from Saturn's moon Titan placed the two-and-a-half ton vehicle on its path for impending destruction. Yet several mission milestones have to occur over the coming two-plus weeks to prepare the vehicle for one last burst of trailblazing science.

"The Cassini mission has been packed full of scientific firsts, and our unique planetary revelations will continue to the very end of the mission as Cassini becomes Saturn's first planetary probe, sampling Saturn's atmosphere up until the last second," said Linda Spilker, Cassini project scientist from NASA's Jet Propulsion Laboratory in Pasadena, California. "We'll be sending data in near real time as we rush headlong into the atmosphere — it's truly a first-of-its-kind event at Saturn."

The spacecraft is expected to lose radio contact with Earth within about one to two minutes after beginning its descent into Saturn's upper atmosphere. But on the way down, before contact is lost, eight of Cassini's 12 science instruments will be operating. In particular, the spacecraft's ion and neutral mass spectrometer (INMS), which will be directly sampling the atmosphere's composition, potentially returning insights into the giant planet's formation and evolution. On the day before the plunge, other Cassini instruments will make detailed, high-resolution observations of Saturn's auroras, temperature, and the vortices at the planet's poles. Cassini's imaging camera will be off during this final descent, having taken a last look at the Saturn system the previous day (Sept. 14).

In its final week, Cassini will pass several milestones en route to its science-rich Saturn plunge. (Times below are predicted and may change slightly; see here for updated times.)

• Sept. 9 — Cassini will make the last of 22 passes between Saturn itself and its rings — closest approach is 1,044 miles (1,680 kilometers) above the clouds tops.

• Sept. 11 — Cassini will make a distant flyby of Saturn's largest moon, Titan. Even though the spacecraft will be at 73,974 miles (119,049 kilometers) away, the gravitational influence of the moon will slow down the spacecraft slightly as it speeds past. A few days later, instead of passing through the outermost fringes of Saturn's atmosphere, Cassini will dive in too deep to survive the friction and heating.

• Sept. 14 — Cassini's imaging cameras take their last look around the Saturn system, sending back pictures of moons Titan and Enceladus, the hexagon-shaped jet stream around the planet's north pole, and features in the rings.

• Sept. 14 (5:45 p.m. EDT / 2:45 p.m. PDT) — Cassini turns its antenna to point at Earth, begins a communications link that will continue until end of mission, and sends back its final images and other data collected along the way.

• Sept. 15 (4:37 a.m. EDT / 1:37 a.m. PDT) — The "final plunge" begins. The spacecraft starts a 5-minute roll to position INMS for optimal sampling of the atmosphere, transmitting data in near real time from now to end of mission.

• Sept. 15 (7:53 a.m. EDT / 4:53 a.m. PDT) — Cassini enters Saturn's atmosphere. Its thrusters fire at 10 percent of their capacity to maintain directional stability, enabling the spacecraft's high-gain antenna to remain pointed at Earth and allowing continued transmission of data.

• Sept. 15 (7:54 a.m. EDT / 4:54 a.m. PDT) — Cassini's thrusters are at 100 percent of capacity. Atmospheric forces overwhelm the thrusters' capacity to maintain control of the spacecraft's orientation, and the high-gain antenna loses its lock on Earth. At this moment, expected to occur about 940 miles (1,510 kilometers) above Saturn's cloud tops, communication from the spacecraft will cease, and Cassini's mission of exploration will have concluded. The spacecraft will break up like a meteor moments later.

As Cassini completes its 13-year tour of Saturn, its Grand Finale — which began in April — and final plunge are just the last beat. Following a four-year primary mission and a two-year extension, NASA approved an ambitious plan to extend Cassini's service by an additional seven years. Called the Cassini Solstice Mission, the extension saw Cassini perform dozens more flybys of Saturn's moons as the spacecraft observed seasonal changes in the atmospheres of Saturn and Titan. From the outset, the planned endgame for the Solstice Mission was to expend all of Cassini's maneuvering propellant exploring, then eventually arriving in the ultra-close Grand Finale orbits, ending with safe disposal of the spacecraft in Saturn's atmosphere.

"The end of Cassini's mission will be a poignant moment, but a fitting and very necessary completion of an astonishing journey," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory in Pasadena, California. "The Grand Finale represents the culmination of a seven-year plan to use the spacecraft's remaining resources in the most scientifically productive way possible. By safely disposing of the spacecraft in Saturn's atmosphere, we avoid any possibility Cassini could impact one of Saturn's moons somewhere down the road, keeping them pristine for future exploration."

posted 09-11-2017 06:05 PM               

Cassini Makes its 'Goodbye Kiss' Flyby of Titan

NASA's Cassini spacecraft is headed toward its Sept. 15 plunge into Saturn, following a final, distant flyby of the planet's giant moon Titan.

The spacecraft made its closest approach to Titan today [Sept. 11] at 12:04 p.m. PDT (3:04 p.m. EDT), at an altitude of 73,974 miles (119,049 kilometers) above the moon's surface. The spacecraft is scheduled to make contact with Earth on Sept. 12 at about 6:19 p.m. PDT (9:19 p.m. EDT). Images and other science data taken during the encounter are expected to begin streaming to Earth soon after. Navigators will analyze the spacecraft's trajectory following this downlink to confirm that Cassini is precisely on course to dive into Saturn at the planned time, location and altitude.

This distant encounter is referred to informally as "the goodbye kiss" by mission engineers, because it provides a gravitational nudge that sends the spacecraft toward its dramatic ending in Saturn's upper atmosphere. The geometry of the flyby causes Cassini to slow down slightly in its orbit around Saturn. This lowers the altitude of its flight over the planet so that the spacecraft goes too deep into Saturn's atmosphere to survive, because friction with the atmosphere will cause Cassini to burn up.

Cassini has made hundreds of passes over Titan during its 13-year tour of the Saturn system — including 127 precisely targeted encounters — some at close range and some, like this one, more distant.

"Cassini has been in a long-term relationship with Titan, with a new rendezvous nearly every month for more than a decade," said Cassini Project Manager Earl Maize at NASA's Jet Propulsion Laboratory in Pasadena, California. "This final encounter is something of a bittersweet goodbye, but as it has done throughout the mission, Titan's gravity is once again sending Cassini where we need it to go."

Cassini is ending its 13-year tour of the Saturn system with an intentional plunge into the planet to ensure Saturn's moons — in particular Enceladus, with its subsurface ocean and signs of hydrothermal activity — remain pristine for future exploration. The spacecraft's fateful dive is the final beat in the mission's Grand Finale, 22 weekly dives (begun in late April) through the gap between Saturn and its rings. No spacecraft has ever ventured so close to the planet before.

posted 09-14-2017 02:03 AM               

Cassini Spacecraft Makes Its Final Approach to Saturn

NASA's Cassini spacecraft is on final approach to Saturn, following confirmation by mission navigators that it is on course to dive into the planet's atmosphere on Friday, Sept. 15.

Cassini is ending its 13-year tour of the Saturn system with an intentional plunge into the planet to ensure Saturn's moons – in particular Enceladus, with its subsurface ocean and signs of hydrothermal activity – remain pristine for future exploration. The spacecraft's fateful dive is the final beat in the mission's Grand Finale, 22 weekly dives, which began in late April, through the gap between Saturn and its rings. No spacecraft has ever ventured so close to the planet before.

The mission's final calculations predict loss of contact with the Cassini spacecraft will take place on Sept. 15 at 7:55 a.m. EDT (4:55 a.m. PDT). Cassini will enter Saturn's atmosphere approximately one minute earlier, at an altitude of about 1,190 miles (1,915 kilometers) above the planet's estimated cloud tops (the altitude where the air pressure is 1-bar, equivalent to sea level on Earth). During its dive into the atmosphere, the spacecraft's speed will be approximately 70,000 miles (113,000 kilometers) per hour. The final plunge will take place on the day side of Saturn, near local noon, with the spacecraft entering the atmosphere around 10 degrees north latitude.

When Cassini first begins to encounter Saturn's atmosphere, the spacecraft's attitude control thrusters will begin firing in short bursts to work against the thin gas and keep Cassini's saucer-shaped high-gain antenna pointed at Earth to relay the mission's precious final data. As the atmosphere thickens, the thrusters will be forced to ramp up their activity, going from 10 percent of their capacity to 100 percent in the span of about a minute. Once they are firing at full capacity, the thrusters can do no more to keep Cassini stably pointed, and the spacecraft will begin to tumble.

When the antenna points just a few fractions of a degree away from Earth, communications will be severed permanently. The predicted altitude for loss of signal is approximately 930 miles (1,500 kilometers) above Saturn's cloud tops. From that point, the spacecraft will begin to burn up like a meteor. Within about 30 seconds following loss of signal, the spacecraft will begin to come apart; within a couple of minutes, all remnants of the spacecraft are expected to be completely consumed in the atmosphere of Saturn.

Due to the travel time for radio signals from Saturn, which changes as both Earth and the ringed planet travel around the Sun, events currently take place there 83 minutes before they are observed on Earth. This means that, although the spacecraft will begin to tumble and go out of communication at 6:31 a.m. EDT (3:31 a.m. PDT) at Saturn, the signal from that event will not be received at Earth until 86 minutes later.

"The spacecraft's final signal will be like an echo. It will radiate across the solar system for nearly an hour and a half after Cassini itself has gone," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "Even though we'll know that, at Saturn, Cassini has already met its fate, its mission isn't truly over for us on Earth as long as we're still receiving its signal."

Cassini's last transmissions will be received by antennas at NASA's Deep Space Network complex in Canberra, Australia.

Cassini is set to make groundbreaking scientific observations of Saturn, using eight of its 12 science instruments. All of the mission's magnetosphere and plasma science instruments, plus the spacecraft's radio science system, and its infrared and ultraviolet spectrometers will collect data during the final plunge.

Chief among the observations being made as Cassini dives into Saturn are those of the Ion and Neutral Mass Spectrometer (INMS). The instrument will directly sample the composition and structure of the atmosphere, which cannot be done from orbit. The spacecraft will be oriented so that INMS is pointed in the direction of motion, to allow it the best possible access to oncoming atmospheric gases.

For the next couple of days, as Saturn looms ever larger, Cassini expects to take a last look around the Saturn system, snapping a few final images of the planet, features in its rings, and the moons Enceladus and Titan. The final set of views from Cassini's imaging cameras is scheduled to be taken and transmitted to Earth on Thursday, Sept. 14. If all goes as planned, images will be posted to the Cassini mission website beginning around 11 p.m. EDT (8 p.m. PDT). The unprocessed images will be available here.

posted 09-15-2017 07:08 AM               

NASA's Cassini mission at Saturn ends in plunge into ringed planet

In the end, Cassini concluded its 13-year NASA mission orbiting Saturn in a meteoritic blaze that likely lasted less than two minutes...

Cassini's final signal was received on Earth by antennas in Canberra, Australia, at 4:55 a.m. PDT (1155 GMT) Friday (Sept. 15), but because it takes 83 minutes for radio waves to travel from Saturn, the spacecraft was already gone.

posted 10-16-2017 02:30 PM               

Fresh Findings From Cassini

NASA's Cassini spacecraft ended its journey on Sept. 15 with an intentional plunge into the atmosphere of Saturn, but analysis continues on the mountain of data the spacecraft sent during its long life. Some of the Cassini team's freshest insights were presented during a news conference today at the American Astronomical Society Division for Planetary Science meeting in Provo, Utah.

Among the findings being shared:

Views from Cassini's Grand Finale show the beauty of the rings and demonstrate processes similar to those that form planets.

During Cassini's final months, the spacecraft's cameras captured views from within the gap between the planet and the rings, and the mission is releasing two new image mosaics showing the rings from that unique perspective. One view, from May 28, 2017, shows the rings emerging from behind the planet's hazy limb, while the planet itself is adorned with ring shadows. The other mosaic shows a panoramic view outward across the ringscape.

Researchers also shared a new movie of Saturn's auroras in ultraviolet light that represents the final such view from the spacecraft's Ultraviolet Imaging Spectrometer.

In addition, Cassini participating scientist and imaging team associate Matt Tiscareno of SETI Institute, Mountain View, California, provided new details about the whimsically named ring features called propellers, which are wakes in the rings created by small, unseen moonlets. The propellers are analogous to baby planets forming in disks around young stars, as they obey similar physical processes.

Tiscareno said that, in its last images of the rings (taken the day before the spacecraft's plunge into Saturn), Cassini successfully imaged all six of the propellers whose orbits were being tracked over the last several years of the mission. These objects are named for famous aviators: Blériot, Earhart, Santos-Dumont, Sikorsky, Post and Quimby. During its Ring-Grazing Orbits -- the four months of close orbits that preceeded the mission's Grand Finale -- Cassini obtained images showing swarms of smaller propellers, astounding Tiscareno and colleagues.

Cassini's electronic "nose" hit the jackpot, finding many surprises as it sniffed the gases in the previously unexplored space between the planet and the rings.

The spacecraft's Ion and Neutral Mass Spectrometer (INMS) returned a host of first-ever direct measurements of the components in Saturn's upper atmosphere, which stretches almost to the rings. From these observations, the team sees evidence that molecules from the rings are raining down onto the atmosphere. This influx of material from the rings was expected, but INMS data show hints of ingredients more complex than just water, which makes up the bulk of the rings' composition. In particular, the instrument detected methane, a volatile molecule that scientists would not expect to be abundant in the rings or found so high in Saturn's atmosphere. Cassini participating scientist and INMS team associate Mark Perry from the Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, says the team is busy analyzing data from the final, lowest-altitude passes, which show even more complexity and variability. The INMS observations complement those by Cassini's Cosmic Dust Analyzer instrument, which sampled solid particles in the gap during the Grand Finale.

Researchers continue trying to wrangle insights about the length of the planet's day from measurements of Saturn's magnetic field.

Michele Dougherty, leader of Cassini's Magnetometer team from Imperial College London, provided an update on the team's progress in trying to determine whether Saturn's magnetic field has a detectable tilt. One aim of their work is to determine the precise length of time for the planet's internal rotation, which would help researchers nail down the true length of the planet's day. Dougherty says the sensitivity of Cassini's magnetic field measurements nearly quadrupled over the course of the spacecraft's 22 Grand Finale orbits -- meaning that, if the tilt of Saturn's field is greater than 0.016 degrees, researchers should be able to detect it. An extremely small tilt is challenging to explain with scientists' current understanding of how planetary magnetic fields are generated, thus suggesting more sophisticated dynamics inside Saturn.

New theoretical reseach explains the forces that keep Saturn's rings from spreading out and dispersing. It turns out to be a group effort.

Key among the questions scientists hope to answer using data from Cassini are the age and origins of the rings. Theoretical modeling has shown that, without forces to confine them, the rings would spread out over hundreds of millions of years -- much younger than Saturn itself. This spreading happens because faster-moving particles that orbit closer to Saturn occasionally collide with slower particles on slightly farther-out orbits. When this happens, some momentum from the faster particles is transferred to the slower particles, speeding the latter up in their orbit and causing them to move farther outward. The inverse happens to the faster, inner particles.

Previous research had shown that gravitational tugs from the moon Mimas are solely responsible for halting the outward spread of Saturn's B ring -- that ring's outer edge is defined by the dark region known as the Cassini Division. Ring scientists had thought the small moon Janus was responsible for confining the outer edge of the A ring. But a new modeling study led by Radwan Tajeddine of Cornell University, Ithaca, New York, shows that the A ring's outward creep is kept in check by a confederation of moons, including Pan, Atlas, Prometheus, Pandora, Janus, Epimetheus and Mimas.

The insight was made possible by Cassini, which provided scientists with high-resolution views of intricate waves in the rings, along with precise determinations of the masses of Saturn's moons. Analysis of these data led Tajeddine and colleagues to an understanding that a cumulative effect of waves from all these moons damps the outward transfer of momentum in the A ring and confines its edge.

Tajeddine will present these results in a poster at the DPS meeting, and they will be published Wednesday in the Astrophysical Journal.

"There are whole careers to be forged in the analysis of data from Cassini," said Linda Spilker, the mission's project scientist at NASA's Jet Propulsion Laboratory, Pasadena, California. "In a sense, the work has only just begun."

posted 04-15-2019 11:04 AM               

NASA's Cassini Reveals Surprises with Titan's Lakes

On its final flyby of Saturn's largest moon in 2017, NASA's Cassini spacecraft gathered radar data revealing that the small liquid lakes in Titan's northern hemisphere are surprisingly deep, perched atop hills and filled with methane.

The new findings, published April 15 in Nature Astronomy, are the first confirmation of just how deep some of Titan's lakes are (more than 300 feet, or 100 meters) and of their composition. They provide new information about the way liquid methane rains on, evaporates from and seeps into Titan - the only planetary body in our solar system other than Earth known to have stable liquid on its surface.

Scientists have known that Titan's hydrologic cycle works similarly to Earth's - with one major difference. Instead of water evaporating from seas, forming clouds and rain, Titan does it all with methane and ethane. We tend to think of these hydrocarbons as a gas on Earth, unless they're pressurized in a tank. But Titan is so cold that they behave as liquids, like gasoline at room temperature on our planet.

Scientists have known that the much larger northern seas are filled with methane, but finding the smaller northern lakes filled mostly with methane was a surprise. Previously, Cassini data measured Ontario Lacus, the only major lake in Titan's southern hemisphere. There they found a roughly equal mix of methane and ethane. Ethane is slightly heavier than methane, with more carbon and hydrogen atoms in its makeup.

"Every time we make discoveries on Titan, Titan becomes more and more mysterious," said lead author Marco Mastrogiuseppe, Cassini radar scientist at Caltech in Pasadena, California. "But these new measurements help give an answer to a few key questions. We can actually now better understand the hydrology of Titan."

Adding to the oddities of Titan, with its Earth-like features carved by exotic materials, is the fact that the hydrology on one side of the northern hemisphere is completely different than the that of other side, said Cassini scientist and co-author Jonathan Lunine of Cornell University in Ithaca, New York.

"It is as if you looked down on the Earth's North Pole and could see that North America had completely different geologic setting for bodies of liquid than Asia does," Lunine said.

On the eastern side of Titan, there are big seas with low elevation, canyons and islands. On the western side: small lakes. And the new measurements show the lakes perched atop big hills and plateaus. The new radar measurements confirm earlier findings that the lakes are far above sea level, but they conjure a new image of landforms - like mesas or buttes - sticking hundreds of feet above the surrounding landscape, with deep liquid lakes on top.

The fact that these western lakes are small - just tens of miles across - but very deep also tells scientists something new about their geology: It's the best evidence yet that they likely formed when the surrounding bedrock of ice and solid organics chemically dissolved and collapsed. On Earth, similar water lakes are known as karstic lakes. Occurring in in areas like Germany, Croatia and the United States, they form when water dissolves limestone bedrock.

Alongside the investigation of deep lakes, a second paper in Nature Astronomy helps unravel more of the mystery of Titan's hydrologic cycle. Researchers used Cassini data to reveal what they call transient lakes. Different sets of observations - from radar and infrared data - seem to show liquid levels significantly changed.

The best explanation is that there was some seasonally driven change in the surface liquids, said lead author Shannon MacKenzie, planetary scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. "One possibility is that these transient features could have been shallower bodies of liquid that over the course of the season evaporated and infiltrated into the subsurface," she said.

These results and the findings from the Nature Astronomy paper on Titan's deep lakes support the idea that hydrocarbon rain feeds the lakes, which then can evaporate back into the atmosphere or drain into the subsurface, leaving reservoirs of liquid stored below.

Cassini, which arrived in the Saturn system in 2004 and ended its mission in 2017 by deliberately plunging into Saturn's atmosphere, mapped more than 620,000 square miles (1.6 million square kilometers) of liquid lakes and seas on Titan's surface. It did the work with the radar instrument, which sent out radio waves and collected a return signal (or echo) that provided information about the terrain and the liquid bodies' depth and composition, along with two imaging systems that could penetrate the moon's thick atmospheric haze.

The crucial data for the new research were gathered on Cassini's final close flyby of Titan, on April 22, 2017. It was the mission's last look at the moon's smaller lakes, and the team made the most of it. Collecting echoes from the surfaces of small lakes while Cassini zipped by Titan was a unique challenge.

"This was Cassini's last hurrah at Titan, and it really was a feat," Lunine said.